This invention relates to the field of optical communications systems.
Most residential communications systems, including connections to the Internet, use twisted-pair, copper wire lines that were designed only for narrowband telephony. Most residential Internet connections rely on, at best, 56 kb/s dial-up modems connected to the copper wire lines. When compared with the speed available from fiber optic lines, the use of the twisted-pair copper wires and/or 56 kb/s modems creates an access bottleneck for the user.
In some areas, optical fiber is being used to replace traditional twisted-pair copper wires; typically, however, this process occurs only at the central network level, no closer to the home than the last switch or access server. Extending fiber deeper into the network can be an important part of relieving the access bottleneck. Many attempts have been made to extend a fiber to the home (FTTH, serving a single living unit), curb (FTTC, serving approximately 16 living units), or cabinet (FTTCab), serving approximately 100 living units). Most involve a Passive Optical Network (PON), which runs one feeder fiber from the central office out to a passive terminal, then distributes the transmitted signals over distribution fibers to each of typically 16-32 optical network units (ONUs). The ONUs convert from optics to electronics at or near the home. PONs reduce cost by sharing the costly central office optoelectronics and feeder fiber over many ONUs.
Much research has been devoted to exploring PON architectures. Recent work has centered on applications of dense wavelength division multiplexing (DWDM). DWDM has been tremendously successful in the long haul arena (e.g., connectivity from one city to another) and is beginning to find applications in metropolitan-area systems (e.g., connectivity within a city). One proposed DWDM-based PON uses a wavelength routing device, such as a waveguide-grating router, at the passive terminal of the PON to provide a single, dedicated wavelength (carrying extremely broadband services) to every ONU. Such a system is described in xe2x80x9cA Survey of Fiber Optics in Local Access Architectures,xe2x80x9d N. J. Frigo, Optical Fiber Telecommunications IIIA, pp 461-522 (1997), incorporated herein by reference. Unfortunately, implementing DWDM PONs presents many technical challenges that will be costly to solve, and at this time it does not appear that the costs can be justified for access applications (i.e., applications which provide connectivity to the home).
An alternative approach that appears more promising is to use an optical power splitter at the passive terminal of the PON. A large international group of service providers and equipment vendors, known as the Full Services Access Network (FSAN) consortium, has been working to create a standardized PON carrying data with baseband transmission in ATM cells using this power-splitting approach. The hope is that standardizing the fiber access systems will lead to economies of scale so that these systems become more affordable. The FSAN is a fully-digital system that operates bidirectionally at 155 Mb/s for FTTH. For FTTC or FTTCab, the downstream bit rate (from the central office or head end to the ONUs) could be increased to 622 Mb/s, while the upstream rate would remain at 155 Mb/s. In these PONs, the bandwidth is shared among 16-32 ONUs.
Despite much technical work and many FTTH/C system trials, actual deployments have been limited, primarily because of the difficulty of generating enough revenue on these networks to justify their high installed-first-cost (the capital outlay required before any revenue can be generated) which is dominated by the civil works necessary to install the new fiber (e.g., digging trenches and stringing cables on telephone poles).
To date, broadcast cable television (CATV) is the only residential broadband service that has been widely implemented. Newer CATV systems are hybrid fiber-coax (HFC) networks, with an optical fiber terminating in a fiber node serving 500-2000 homes followed by an extensive coaxial network. CATV networks have been used, in the past, as downstream networks for delivering analog television, and they are optimized for delivering this service economically. Digital video has recently been added to many CATV networks, and two-way services (e.g., cable modems, telephony, pay-per-view video) are beginning to be added as well.
However, the extensive coaxial network is problematical for many advanced applications. Since the coax network is essentially a shared bus, any noise ingress or nonlinearity can detrimentally affect many customers. Even a small degradation in any of the connectors, taps, drop cables, or in-home wiring can produce an opportunity for system-wide noise ingress and other problems. Even with fiber nodes serving fewer than 500 homes, the signal attenuation in the coaxial plant requires several radio-frequency (rf) amplifiers in series between the fiber node and the farthest customer in order to provide a signal of adequate strength. Because coax loss increases with frequency, increasing the system bandwidth requires more amplifiers, and each amplifier adds noise and distortion to the transmitted signal. Thus, practical system bandwidths are limited to approximately 550-860 MHz. Furthermore, the rf amplifiers must be powered, and carrying the power on the coax can accelerate its corrosion. Even in a xe2x80x9ccleanxe2x80x9d coax plant that has been carefully engineered and maintained so as to minimize ingress noise, the available bandwidth for return of signals from the home to the head-end is limited.
An advance is made over the prior art in accordance with the principles of the present invention directed to the integration of signals carrying broadcast CATV services into a FTTH/C network that also provides high-speed Internet access and telephony. Essentially all residential telecommunications services are provided in a single network, based on a single fiber and a single simple transceiver in an optical network unit. Multiple services (e.g., analog video, digital video, cable-modem based Internet access) are multiplexed using separate rf subcarriers (subcarrier multiplexing or SCM) and the delivered signals are compatible with existing consumer appliances (e.g., TVs, VCRs, cable modems, etc.).
According to a first embodiment of the present invention, a method is provided for delivering CATV and data signals from a headend to a service subscriber, comprising the steps of combining the CATV and data signals from the headend into a composite optical signal; transmitting the composite optical signal downstream over a passive optical network (PON) using coarse wavelength division multiplexing (CWDM) for duplexing; converting the transmitted composite optical signal to electrical signals; and routing the electrical signals to the service subscriber for use.
Viewed from another aspect, the present invention is directed to an apparatus for delivering CATV and data signals from a headend to a service subscriber, comprising an optical combiner connected to receive the CATV and data signals from the headend; a passive optical network (PON) connected to an output of the transmitter; and an optical-electrical converter (OEC) connected to the PON, wherein the CATV and data signals are combined into a composite downstream optical signal and transmitted via the PON to the OEC and then delivered to the service subscriber for use. In a preferred embodiment the OEC is located on the subscriber""s premises, and the upstream signals are received from the subscriber over the PON and routed over a dedicated upstream network to the headend in a different wavelength band than the downstream signals.
Further features and advantages of the present invention, as well as structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings.